A New Approach to Sediment Transport in the Design and Operation of Irrigation Canals

UNESCO-IHE Lecture Note Series

Herman Depeweg, Néstor Méndez V

• Publisher: CRC Press
• Year: 2007
• eBook ISBN: 9781134090891
• Topic: Physical Sciences
• Subtopic: Environmental Science

The transport of sediment greatly influences the sustainability of an irrigation system. Erosion and deposition not only increase maintenance costs, but may result in an inequitable and inadequate distribution of irrigation water. Understanding the behavior and transport of sediment allows efficient planning and reliable water delivery schedules, and ensures the controlled deposition of sediments, making maintenance activities more manageable. This book presents a detailed analysis of sediment transport in irrigation canals, together with physical and mathematical descriptions of the behavior. The authors use a mathematical model to predict the sediment transport, deposition and entrainment rate for various flow conditions and sediment inputs. The model is particularly suitable for the simulation of sediment transport in irrigation canals where flow and sediment transport are determined by the operation of flow control structures.

CHAPTER 1
Introduction

The transport of sediment in irrigation canals influences to a great extent the sustainability of an irrigation and drainage system. Unintentional or unwanted erosion or deposition of sediment in canals will not only increase the maintenance costs, but also leads to an unfair and inadequate distribution of irrigation water. Proper knowledge of the behaviour and transport of sediment in these canals will help to plan efficient and reliable water delivery schedules to supply water at the required levels; to have a controlled deposition of sediments, to estimate and arrange the required maintenance activities, and to determine the type of desilting facilities and their efficiency, etc.

The study of sediment transport in irrigation canals is mainly focused on the sediment and erosion processes in canal networks. In view of maintenance activities the head works should be designed in such a way that they prevent or limit the entrance of sediment into canals. In addition, the design of the canal system should be based upon the transport of all the sediment to the fields or to specific places in the canal system, where the deposited sediment can be removed at minimum cost. Sedimentation should be prevented in canals and near structures, as it will hamper and endanger a correct irrigation management, the main objectives of which are to deliver irrigation water in an adequate, reliable, fair and efficient way to all the farmers at the required water level. Inadequate management will result in low efficiency, unnecessary loss of the already scarce water and low yields.

Irrigation canals are usually designed upon the assumption that the water flow is uniform and steady and that the canals are able to carry the water and sediments to the fields. The design supposes that an equilibrium situation exists where the sediments and water entering into the irrigation network will be transported to the fields without deposition or erosion. However, a perfectly uniform and steady flow is seldom found. In the operation of irrigation systems the flow is predominantly non-uniform, with varying discharges and with a constant water level at the regulation points where the water is supplied to the offtakes. The sediment transport capacity of the canals greatly depends on the flow conditions and is variable. Although the water flow can be modelled with a high degree of accuracy, sediment transport is only understood to a limited extent. The predictability of sediment transport equations and models in view of the quantity of sediment that needs to be removed is still rather poor. Computations of the effects of the non-equilibrium flow conditions on sediment transport are required to determine whether deposition and/or entrainment will occur and to assess the amount and distribution of the sediment deposition and/or entrainment along the canals. Mathematical modelling of sediment transport offers the possibility of estimating the distribution of sediment deposition or entrainment rates for a particular flow and a specific situation.

The main criterion for a canal design is the need to convey different amounts of water at a fixed level during the irrigation season in such a way that the irrigation requirements are met. Furthermore, the design must be compatible with the sediment load of a particular location in order to avoid silting and/or scouring of the canal. The water supply should meet the irrigation requirements and at the same time it should result in the least possible deposition in and/or scouring of the canals. The design process becomes more complicated when canals are unlined and pass through alluvial soils.

The problems of design and maintenance of stable channels in alluvial soils are fundamental to all irrigation schemes (Raudkivi, 1993). Stable or regime canals present ideal conditions for non-scouring and non-silting throughout the irrigation network after one or several seasons of operation. The search for the main characteristics of stable channels started with the work of Kennedy between 1890 and 1894 (Mohammad, 1997). Subsequently, different theories have been developed and are used around the world. All of them assume uniform and steady flow conditions and try to find those canal dimensions that are stable for a given discharge and sediment load.

In the past the irrigation canals used to be designed for protective irrigation, where the government or an irrigation authority ran and maintained the system. The available water was spread over as large an area as possible. The canals carried an almost constant discharge without significant control structures and to some extent the assumption of a uniform and steady flow was realised. In these situations a prediction of the behaviour of the sediment in the canal could be made with some reliability. The growing need for reducing the continually increasing governmental investments in irrigation demand a more economical system, so that the farmers can pay for the operational and maintenance costs with the return that they get from irrigated agriculture. In addition, farmers would like to have a more reliable and flexible water delivery. Flexibility in the delivery system demands more frequent regulation of the water flow, which will create unsteadiness in the flow. This unsteadiness will make all the assumptions made in the original design imprecise and reduces the accuracy of the already poor sediment transport predictors.

Generally three methods for the design of irrigation canals are used: namely Lacey’s regime method and the tractive force method for large irrigation schemes, and the permissible velocity method for irrigation systems in hilly areas. Sediment characteristics and coefficients are either simply borrowed from literature or chosen based on the experience of the designer. Hence, there exists a large difference in the design parameters from scheme to scheme and even from canal to canal within the same scheme. Without incentives or obligations for any verification, most designers do not investigate and evaluate the performance of their design in the field.

At the moment, the improvement of the performance of existing schemes is more pressing than the development of new irrigation systems, especially in view of the high investment costs required for the construction and operation of new systems. Appropriate management of sediment in canal networks is one of the major challenges of the improvement works, as a major part of the available maintenance budgets is spent annually on the removal of the sediment deposited. This type of schemes imposes extra conditions on the designer as the canal slope, bed width, structure control, and management practices already exist and in most cases they cannot be changed due to economic resources and/or social considerations. Hence, the selection of an appropriate design philosophy and its applicability for these particular conditions is very important.

The design of irrigation canals is not as simple as normally perceived. It is the final product of a merge of complex and undetermined parameters such as water flows, sediment load, structure control and operation, and management strategies. No design packages are available that deal with all the parameters at the same time. To simplify the design process some parameters are either disregarded or assumed to be constant, which consequently will lead to a less adequate design. Many failures and problems are caused by a design approach that pays insufficient attention to the operational aspects (FAO, 2003). Considering the aforementioned parameters involved in the canal design and their importance in view of the sustainability of the system, a numerical modelling may be one option that can simultaneously simulate all the variables. However, the selection of a model to represent the system and its validity in the proposed environment will have a major influence on the results.

Developments in the knowledge of sediment transport in open canals have mainly been derived from natural channels such as rivers. So far sediment transport theories, the development of bed forms, resistance factors, etc. have been developed under assumptions applicable to the particular conditions encountered in rivers. Even though certain similarities between rivers and irrigation canals exist, the sediment concepts are not entirely applicable to irrigation canals. Most of these irrigation canals are man-made and the irrigation environment presents a number of typical problems that are rarely encountered in rivers. The need to control water levels and discharges in the upstream and/or downstream direction, the necessity to find an optimal cross section and the large influenc...

Table of contents

• UNESCO-IHE LECTURE NOTE SERIES
• Table of Contents
• List of Figures
• List of Tables
• CHAPTER 1 Introduction
• CHAPTER 2 Open Channel Flow
• CHAPTER 3 Sediment Properties
• CHAPTER 4 Design Criteria for Irrigation Canals
• CHAPTER 5 Sediment Transport Concepts
• CHAPTER 6 SETRIC, a Mathematical Model for Sediment Transport in Irrigation Canals
• CHAPTER 7 The Sediment Transport Model SETRIC and its Applications
• References
• Symbols
• APPENDIX A Methods to Estimate the Total Sediment Transport Capacity in Irrigation Canals
• APPENDIX B Methods to Predict the Friction Factor
• APPENDIX C Hydraulic Design of Irrigation Canals
• APPENDIX D Description of the Main Aspects of the Regime Theory
• APPENDIX E Glossary Related to Sediment Transport
• Index